Zener diode incorporating an ion implanted layer establishing the breakdown point below the surface

ABSTRACT

A zener diode in which the anode region of a first conductivity material is formed by diffusion in a semiconductor body, a cathode region of a second conductivity material is formed by diffusion in the semiconductor body, and the two regions are bridged by a third region extending through the two regions, the third region being a shallow layer of ion implanted doping material of said first conductivity type. In one embodiment, the anode and cathode regions are spaced-apart; in a second embodiment the cathode region is formed within the anode region. The ion implanted layer has a concentration that peaks below the surface, thus establishing the breakdown point for the avalanching of the zener diode below the surface and removed from surface contaminants such as found in the oxide surface layer.

This is a division of application Ser. No. 377,610 filed July 19, 1973now U.S. Pat. No. 4,079,402.

BACKGROUND OF THE INVENTION

One conventional manner of fabrication of a zener diode on an integratedcircuit is to form the zener diode during the same processing steps usedto form the bipolar transistors on the silicon wafer. Referring to FIGS.1 and 2, the P+ anode region 11 of the diode is formed in the Nepitaxial region 12 during the base diffusion of the bipolar transistorand the cathode region 13 is formed during the emitter diffusion, thediode structure also including the N+ buried layer region 14 and theisolation regions 15. At the time of diffusion of the cathode region 13,an N+ contact region 16 is diffused into the N epitaxial layer 12. Ametallic interconnect 17 extending over the oxide layer 18 connects theregion 13 with the epitaxial layer contact 16; a metal contact 19 isalso provided for the anode region 11. The contact 17 is connected tothe positive source of voltage supply and contact 19 is connected to thenegative side.

As the voltage across terminals 17 and 19 increases, a point is reachedwhere the zener diode avalanches and the current rapidly increases fromzero to some maximum level. A typical voltage at which avalanche occursis 6-7 volts.

One major problem with such forms of zener diode is that the voltage atwhich the avalanche occurs drifts with time. This is due to the factthat the junction avalanche initially starts at the point where thejunction 20 abutts the oxide layer 18, i.e. at the surface of thesemiconductor body. There is a fringing of the electric field at thispoint along the surface and any factor that influences this electricfield will influence the zener voltage. The oxide will usually containcontaminants, for example sodium atoms; these sodium atoms have a pluscharge and are very mobile in the oxide 18 even at room temperature. Ifthese sodium atoms come near the junction 20, they bend the depletionlayers at the surface and affect the breakdown voltage. Therefore, thebreakdown voltage is a function of the electric field at the surface andmay increase as a function of time, and the circuit may drift out ofspecification existing at first use.

A second disadvantage is that the zener voltage is dependent on thediffusion depths and other parameters of the diffused areas 11 and 13,and these in turn are a function of the deposition steps used to makethe other transistors on the IC. Therefore the zener diodespecifications are principally determined by considerations other thangood zener diode parameters.

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention provides a novel zener diode structure and methodof fabrication wherein an ion implanted layer is used to establish theavalanche junction of the diode well below the surface of the device,whereby contaminants in the oxide layer at the surface have no effect onthe avalanche voltage. The ion implantation technique permits the peakof the impurity concentration of the implant to occur below the surface,and breakdown occurs at this point of peak concentration and well belowthe surface.

In one embodiment of the invention, a first diffusion is made in theepitaxial region of a semiconductor body, for example a P type diffusionin an N epitaxial region during the base diffusion step in forming NPNtransistors, and thereafter a second diffusion is made of a differentconductivity type. This second diffusion is separated a distance fromthe first diffusion; in the particular example this second diffusion isan N+ diffusion made during the emitter diffusion step for the othertransistors. After these two diffusions, a window is opened in asuitable mask which bares the two diffusion areas and the epitaxialregion between the two depositions. A doping material of the firstconductivity type, e.g. P type boron, is then made in the exposed regionby the known ion implantation technique, the peak of concentration ofsuch dopant occurring well below the surface of the semiconductor body.This layer of dopant extends into each of the two deposition regions andacross the space therebetween. The two diffusion areas are each providedwith associated metallic contacts and the device forms a zener diode.The avalanching for this zener diode occurs well below the surface andat the point of peak dopant in the ion implantation layer.

In a second embodiment, the deposition of the first conductivity typematerial is made over the entire area of the zener diode, and thedeposition of the second material is made in one area of the firstdiffusion area. Thereafter, the buried layer of the first conductivitymaterial is made in the first diffusion area and encompassing the seconddiffusion area. This buried layer is formed by the ion implantationtechnique and the peak concentration of this layer occurs well below thesurface of the semiconductor body. The zener diode avalanching occurs atthis peak concentration point and therefore occurs well below thesurface of the semiconductor and unaffected by contaminants in thesurface oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-section and plan views, respectively, of atypical prior art zener diode.

FIGS. 3 and 4 are cross-section and plan views, respectively, of oneembodiment of the zener diode of the present invention.

FIG. 5 is a plot of the impurity profile below the surface for thedevice of FIGS. 3 and 4 taken along section line 5--5 in FIG. 4.

FIGS. 6 and 7 are cross-section and plan views, respectively, of asecond embodiment of the zener diode of the present invention.

FIG. 8 is a plot of the impurity profile for the device of FIGS. 6 and 7taken along section line 8--8 in FIG. 7.

FIG. 9 is a cross-section view of a device similar to that of FIG. 3showing a second ion implanted top layer.

FIG. 10 is a cross-section view of a device similar to that of FIG. 6showing a second ion implanted top layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 3 and 4, one embodiment of the novel zener diodeof the present invention comprises a semiconductor having an N epitaxiallayer 12 on a P substrate with an N+ buried layer 14 therein. During thebase diffusion for the other transistors on the semiconductor body, theP type diffusion region 21 which forms the anode is made in theepitaxial region 12. Thereafter and during the subsequent emitterdiffusion step for forming the other transistors, an N+ diffusion area22 which forms the cathode is made in the N epitaxial region 12 spacedfrom region 21. A suitable mask is then made to produce an openingoverlapping the regions 21 and 22 as shown by the dotted line in FIG. 4and then a buried layer 23 is formed in this region by the well knowntechnique of ion implantation (see, for example the article by J. F.Gibbons, Proc. of IEEE, Vol. 56, page 295, 1968). Suitable electricalcontacts (not shown) are then made with area 21 and area 22 by wellknown metallization when forming the other contacts and interconnectsfor the semiconductor.

An ion implantation apparatus of known type includes a means forapplying a high voltage alternating electrical field, for example, 15KEV, on a gas containing the doping atom desired, for example boron inthe gas BH₂, to ionize the boron in the gas. The gas is acceleratedthrough a mass separator including a magnetic field to separate out theionized boron atoms and direct them through a linear accelerator with anaccelerating potential of from 20 KEV to 150 KEV. Higher energies canalso be used to advantage if made to be compatible with the processtechnology. The beam of positive boron atoms exits the accelerator andis swept over the silicon wafer to implant the boron ions in the region23 defined by the opening in the mask. The beam of ionized boron atomscan be monitored very accurately so that the amount of boron atomsimplanted and also the exact depth of the implant layer can becontrolled very accurately by proper selection of the acceleratingvoltage. The impurity profile of the boron peaks at a point D below thesurface as illustrated by the curve 24 in FIG. 5. The concentration (N)of the P implant is lower than the N+ concentration (curve 25) at depthD as shown in FIG. 5, and the zener breakdown occurs at depth D, whichis typically 0.5μ below the surface, and at the junction 26 of the N+region 22 and the layer 23 as viewed in FIG. 3. A typical concentrationfor the N+ diffusion is 10²¹ /cm³ while the peak of the P implant isabout 10¹⁸ to 10²⁰ /cm³. The N epitaxial concentration is about 10¹⁵ or10¹⁶ /cm³. Another way of stating the concentration of the P implant isby the Q number in atoms/cm², i.e. the total number of atoms in a columnone cm by one cm extending vertically through the P layer, and this istypically 10¹³ to 10¹⁵ atoms/cm².

Another embodiment of the invention is shown in FIGS. 6 and 7 whereinthe P diffusion 28 is made over the entire region to be occupied by theP implant layer 29 and the N+ plug 31. After the P diffusion 28, the N+plug is diffused into a portion of region 28 and then the layer 29 of Ptype material is made by the ion implant technique. The profile of thedopants is shown in FIG. 8 as taken along section line 8--8 in FIG. 7.The concentration of P in area 28 is shown as trace 32, theconcentration of N+ plug 31 is shown as trace 33, and the concentrationof the ion implanted layer is shown as trace 34. It is seen that the ionimplantation concentration peaks below the surface and the zenerbreakdown occurs at point 35 as shown in FIG. 6, i.e. about 0.5μ belowthe surface.

Because of the light doping of the layer 23 of FIGS. 3 and 4 and thelayer 29 of FIGS. 6 and 7, the impurities in the oxide layer overlayingthe anode and cathode regions tend to produce instabilities in thedevice, including noise. It has been discovered that, by following thefirst ion implant layer with a second ion implant of the oppositeconducting type material in a layer which peaks at a shallower depth,the noise of the device is substantially improved. In the examples givenof a first ion implant of P type material, the second ion implant is ofN type material, for example phosphorous.

The device of FIG. 3 is shown in FIG. 9 with this second ion implanted Nlayer 41 incorporated therein. The concentration of this layer is shownby the dotted line 42 in FIG. 5. The concentration of this layer peaksat about 0.2μ and has a concentration of for example, 1×10¹³ to 1×10¹⁴atoms/cm².

A similar ion implanted layer, when incorporated in the device of FIG. 6provides the device of FIG. 10; the concentration curve for this layer43 is shown by the dotted line 44 in FIG. 8.

These shallow ion implanted top layers are further described and claimedin U.S. patent application Ser. No. 377,611 filed July 9, 1975, nowabandoned by James Dunkley and Robert Dobkin entitled "SemiconductorDevice with an Ion Implanted Stabilizing Layer".

What is claimed is:
 1. The method of fabricating a zener diode structurecomprising the steps of forming an anode region of a first conductivitytype material in a semiconductor body, forming a cathode region of asecond conductivity type material in said semiconductor body, andforming a third region comprising a shallow layer of ion implanteddoping material of said first conductivity type contiguous with bothsaid anode region and said cathode region.
 2. The method of fabricatinga zener diode structure as claimed in claim 1 including the step ofcontrolling the depth and the height of said third region and the amountof doping by controlling the beam of ions used for the ion implantation.